Yeah, seriously- S2000 or 350z. Light + nimble vs import muscle, same quarter mile... I've beaten the crap out of the S2k for 75k miles and it's over 100k now- no problems other than one coil pack at 80k ($40), TPS replacement (now, $65), and roughing up the timing chain tensioner (normal at this mileage, $0). It's not ridiculously fast, but it's always really damn entertaining. Every time I drive it, I still love it. Just keep feeding with a steady (but light) diet of tires, oil, gas and brake pads and it's super happy.
My primary suggestion:
Replace clutch - if I can do a pain in the ass FWD Mazda in one night in my driveway with crappy lighting and broken engine mounts threatening to drop it on my head and no lift, you can do a RWD car. MUCH easier.
Figure out what's going on with the detonation. It's probably injectors or spark plugs, something simple.
This is what I'd get in your price range:
E36 chassis 3 series with Vorshlag V8 swap (reliable, can be built for <$20k)
Miata
S2000
350z
RX-8
G35 sedan + stick (old roomie sold his cherry model for $14k)
VW GTI
dveit- I had some pretty big guys in my S2000 without any problems. 6'4" muscled out dudes fit pretty well.
I would NOT get a 335. Go look up fuel system problems. Not fun.